Electric discharge machining method and apparatus

ABSTRACT

Upon electric-discharge machining a workpiece by intermittently applying pulse voltages across a gap between it and the opposed electrode, a pause time following each pulse voltage is adjusted by predetermined small incremental amounts in accordance with the voltage conditions developed across the gap.

United States Patent [151 3,670,136 Saito et a1. [4 June 13, 1972 [54]ELECTRIC DISCHARGE MACHINING [56} References Cited METHOD AND APPARATUSUNITED STATES PATENTS [72] Inventors: Nagao Saito; Kazuhlko Kobayashi,both of Nagoya Japan 1,061,612 5/1913 Heroult ..l3/l 3 3,264,517 8/1966Ullman et a1. ..2l9/69 P Asslgnw Mlls'lbishl Kabushfld K8181, 3,439,1454/1969 Sennowitz ..2l9/69 P Chiyoda-ku, Tokyo, Japan [22] Filed: Feb. 2,1970 Primary ExaminerR. F. Staubly 1 pp NOJ 7,898 Attorney-Robert E.Burns and Emmanuel J. Lobato [57] ABSTRACT [30] Foreign ApplicationPnomy Dam Upon electric-discharge machining a workpiece by intermit-Feb. 4, 1969 Japan ..44/8326 tently applying pulse voltages across a gapbetween it and the opposed electrode, a pause time following each pulsevoltage [52] U.S. Cl ..2l9/69 C, 219/69 S is adjusted by predeterminedsmall incremental amounts in ac- [51] InLCl ..B23p l/08 -da with thvolta e conditigns developed across the [58] Field oISearch..13/l3;2l9/69 C, 69 F, 69 G, gap,

7 Claims, 8 Drawing Figures Ame wom ELECTRIC DISCHARGE MACHINING METHODAND APPARATUS BACKGROUND OF THE INVENTION This invention relates to anelectric discharge machining method of and apparatus for machining aworkpiece by intermittently applying pulse voltages across a working gapformed between the workpiece and the opposed working electrode with eachpulse voltage followed by a pause time.

It is well known that in electric discharge machining methods comprisingthe step of applying across the working gap pulse voltages these pulsevoltages are of the important electric requirements for determining theelectric discharge machining characteristics. The requirements thatshould be met by the pulse involve a peak valve of a discharge currentdue to the pulse voltage, the pulse width of the voltage and a pausetime following each pulse voltage. Among these requirements the peakvalue of the discharge current and the pulse width of the voltagedirectly affect the machining characteristics such as the roughness ofthe surface to be finished determined for the particular purpose, aratio of consumption of a working electrode involved relative to anamount of workpiece material removed, a clearance or a lateral gapbetween the electrode and the workpiece etc. while the pause timeaffects the machining efficiency. Actually, the peak value of thedischarge current and the pulse width of the pulse voltage aredefinitely determined by the machining characteristics inherent to amachining apparatus involved and to the initiation of the particularoperation. However it has been common practice to preset suitably thepause time by the operator by deducing how much the pause time mayshorten, that is to say, how much the machining efficiency may increasein accordance with the particular machining conditions, for example, thematerial of the workpiece, the material and configuration of the workingelectrode, the depth to which the workpiece is machined whether or notthe working liquid is spouted into the working gap, etc. and on thebasis of experience.

If the pause time is preset to be short the duty factor and thereforethe mean working current will increase. This generally leads to anincrease in machining efficiency. However, if the pause time is presetto be too short then an abnormal electric discharge it apt to occur andan unexpected result may be encountered. Thus the proper setting of thispause time is difficult and requires the operator to become considerablyskillful.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention toprovide a new and improved method for effecting electric dischargemachining while the machining efficiency is always maintained high inoperation.

It is another object of the invention to provide a new and improvedmethod for effecting electric discharge machining with an efficiencyalways maintained high while a machining gap involved is effectivelyprevented from discharging abnormally.

The invention also has for its other objects to provide a new andimproved electric discharge machining apparatus capable of accomplishingthe objects as described in the preceding paragraphs.

The invention accomplishes the aforesaid objects by the provision of anelectric discharge machining method comprising the steps of disposing aworkpiece and a working electrode in opposed relationship to formaworking gap therebetween, and intermittently applying across the workinggap pulse voltages each followed by a pause time, the pulse voltagesproducing electric discharges for electric discharge machining theworkpiece, characterized by the steps of sensing the state of theworking gap on the basis of the presence of the electric dischargedeveloped across the working gap during the application of each pulsevoltage thereacross, and adjusting the pause time in accordance with thesensed result.

The presence of a normal state in the working gap may be determined by asequence of a predetermined number of pulse voltages each including apulse portion causing no electric discharge across the working gapduring the application of each pulse voltage thereacross and the pausetime decreases by a predetermined small amount. On the other hand, thepresence of an abnormal state in the working gap may be determined by asequence of a predetermined number of pulse voltages each causing anelectric discharge across the working gap throughout the period ofcorresponds application and the pause time increases by a predeterminedgap amount. In this way the pause time may be adjusted by thepredetermined small incremental amounts.

In a preferred apparatus of the invention, there may be provided a firstswitching element for intermittently interrupting the power supply tothe working gap to intermittently apply across the working gap pulsevoltages each followed by a pause time, and adjusting means foradjusting the pause time in accordance with a voltage developed acrossthe working gap.

The adjusting means may advantageously respond to a second switchingelement having a pair of operative positions one of which corresponds tothe absence of electric discharge across the working gas and the otherof which corresponds to both the presence of the electric dischargeacross the gap and the pause time.

BRIEF DESCRIPTION OF THE DRAWINGS 'The invention will become moreappararent from the following detailed description'taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of an electric discharge machiningapparatus embodying the principles of the invention;

FIG. 2 is a logic flow chart useful in explaining the manner in whichthe pause time is adjusted by predetermined small incremental amounts;

FIG. 3 is a waveform diagram illustrating waveforms of voltage andcurrent'developed between the workpiece and the working electrode shownin FIG. 1;

FIG. 4 is a circuit diagram of a state-of-gap sensor and a pulsegenerator constructed in accordance with the principles of theinvention;

FIG. 5 is a schematic circuit diagram of a modification of thearrangement shown in FIG. 4;

FIG. 6 is a view illustrating waveforms developed in the arrangementshown in FIG. 5;

FIG. 7 is a graph plotting the mean machining current against time forthe arrangement shown in FIG. 5; and

FIG. 8 is a graph plotting various machining parameters against time forthe apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand FIG. 1 in particular, it is seen that an arrangement disclosedherein comprises a machining vessel 10 open at the upper end, a quantityof any suitable electrically insulating oil l2,such as kerosene, fillingthe vessel 10, a workpiece l4 suitably disposed within the insulatingoil 12 in the vessel and a working electrode 16 movably suspendend fromabove and dipped into the insulating oil 12 with the lower end facethereof opposed to the workpiece 12 to form a working gap 18thcrebetween. The working electrode 16 is supported by a supportmechanism generally designated by the reference numeral 20.

As the electric discharge machining process proceeds the supportmechanism 20 serves to move the electrode 16 toward the workpiece 12 inthe well known manner to maintain the machining gap 20 substantiallyconstant. The support mechanism 20 is shown in FIG. 1 as being of ahydraulic servo system including a hydraulic cylinder 20a, a piston 20bslidable within the cylinder, and a connecting rod 20c connecting thepiston 20 to the working electrode 16. In order to control the hydraulicpressures within the cylinder 20a on both sides of the piston 18b thecylinder 20a is operatively coupled on both end portions to a control18d as shown in FIG. 1. Such a hydraulic servo system is well known inthe art and need not be described in detail. If desired, the hydraulicservo system may be replaced by any suitable electric servo system.

The control 20d is operatively associated with an electric controlcircuit generally designated by the reference numeral 322. The controlcircuit 22 may be of conventional construction as is shown in FIG. 1 asincluding a potentiometer 22a connected across the electrode 16 and theworkpiece 14 to provide a detection signal V indicating the voltageacross the gap 18, a source of direct current 22b having one terminal,in this case the negative terminal, connected to the workpiece 14, and areference potentiometer 220 connected across the source 22b to provide areference voltage Vs. A servo coil 22d is connected between the movabletaps on both the potentiometers 22a and c through a series resistor 22cthereby to provide a difference signal corresponding to a differencebetween the detection and reference voltages V and Vs respectively. Ifdesired, the control circuit may of any suitable type other than thetype illustrated. For the present purpose, it is sufficient that theservomechanism 20 cooperates with the control circuit 22 to maintain themachining gap 20 substantially constant in operation.

In order to supply electric energy to the working gap 18, a three-phasepower transformer generally designated by the reference numeral 24 isprovided including three primary windings 24a connected in a starconfiguration and three secondary windings 24b also connected in a starconfiguration. The primary windings 24a are adapted to be energized by athree-phase source of alternating current (not shown) having commercialfrequency, and the secondary windings 24b are connected to alternatingcurrent (AC) input terminals of a three-phase full-wave rectifier 26.The rectifier 26 has one direct current (DC) output terminal connectedto the connecting rod 20c and hence the electrode 16 through a reactor28 and the other output terminal connected to the junction of thereactor 28 and the rod 200 through a capacitor 30 relatively high incapacitance. The reactor 26 and the capacitor 30 form a filter circuit32 to smooth the rectified output from the rectifier 26. In theembodiment illustrated the positive side of the filter circuit 32 isconnected to the working electrode 16 through the connecting rod 200 andthe negative side thereof is connected to the workpiece 14'through atransistorized switching element generally designated by the referencenumeral-34. i

As shown in FIG. 1 the'switching element 34 comprises a plurality ofNPN-type transistors 34a, b, ,m including respective emitter electrodesconnected together to the negative side of the filter circuit 32,respective collector electrodes connected together to the workpiece 14through respective collector resistors 36a, b, m and respective baseelectrodes connected together through respective base resistors. Thecollector resistors 36a, b, m serve to maintain the associated collectorcurrents under their rated values as well as balancing those currents.The switching element 34 as above described serves to intermittentlypass the DC output from the filter circuit 32 therethrough to provide atrain of square pulse voltages in the manner as will be describedhereinafter.

' The number of the parallel transistors 34 generally depends upon themagnitude of current flowing through the working I gap 18 and may besmall for a low current flowing through the gap 18. For example, asingle transistor may be used.

A state-of-gap sensor 38 is connected across the workpiece 14 and theconnecting rod 20c and therefore the working electrode 16 to sense avoltage across the working gap 18 thereby to determine if the particularelectric discharge across the gap is normal. If an abnormal dischargehas been determined to occur in the working gap l8 the sensor 38provides an instruction for setting intervals between the pulse voltagesto be long. On the other hand, after the detector 38 has determined thatnormal discharge has continued for a predetermined period of time, itprovides an instruction for setting the intervals convert the dc'outputfrom the filter circuit 32 to a train of square pulses each followed bya controlled pause time. Then the train of square pulses is appliedacross the working gap 18 of the electric discharge machine and theworkpiece 14.

Upon initiating a machining operation, the peak value and pulse width ofa discharge voltage is first preset in accordance with the machiningconditions such as the roughness of a surface to be finished, aclearance, a ratio of consumption of an electrode involved to an amountof workpiece material removed etc. preliminarily determined for theparticular purpose. The peakvalue and pulse width of the dischargevoltage remain unchanged within a limited machining time. According tothe invention an initial pause time is first given and then the pausetime is adjusted by predetermined amounts in accordance with the voltageacross the working gap. Such discrete pause times are each designated bythe reference character 11', where i is any integer. It is assumed thatthe greater the i the longer the pause time 1,.

The pause time can be controlled in accordance with a logic flow chartas shown in FIG. 2. Specifically, an electric discharge machiningapparatus such as shown in FIG. 1 is started in block 100 while thesuffix i of the 1- is set to an integer k indicating the particularinitial p value of the pause time in block 102. Then the particularpause time in' this case 1,, is determined in block 104. In block 106the state sensor 38 determines whether the pause time thus determinedputs the working gap 18in its normal or abnormal state as will bedescribed hereinafter. If the presence of the abnormal state has beendetermined in block 106, block 108 is operated to add one to the pausenumber i in this case k. Then a pause time with the sufiix increased byone is determined in block 104 and the machining operation is nowperformed with the pause time increased by a predetermined incrementalamount. Due to the fixed pulse width this increase in pause time causesa decrease in duty factor leading to a decrease in mean working current.Therefore, the machining operation tends to approach the normal mode.However, if with the increased pause time, the abnormal mode ofoperation still continues as determined in block 106 the process asabove described is repeated to further increase the pause time by apredetermined incremental amountrln this way the pause time increases bypredetermined small amounts until the abnormal state of the gapdisappears.

After the abnormal state has been removed or if the block 106 hasdetermined that the normal state is present in the working gap 18 thenthe block 110 begins to time a predetermined time interval of T thenormal mode. When it has timed out the time interval of T for which gap18 continued to be put in its normal state, the process goes to blockll2 where i decreases by one. Then block 104 determines a pause time 1'decreased by a predetermined amount. Block 106 again determines if thenow decreased pause time TH puts the working gap 18 in its abnormalstate. If the abnormal state takes place in the gap 18 as determined byblock 106, then blocks 108 and 104 return the pause time back to thejust preceding value. On the contrary, if block 106 determines thepresence of the normal state, block 112 again begins to time the timeinterval of T When the normal state has continued for that interval of Tas determined by block 1 12, the pause time is further decreased by apredetermined amount in the same manner as above described. In this waythe machining operation continues to be performed until a predeterminedarea of the workpiece is completed. At that time the operation ends inblock 114.

From the foregoing it will be appreciated that even with the machiningconditions varied, a trial and error operation as above described inconjunction with FIG. 2 can be repeated to automatically select aparticular pause time providing always the optimum machining conditions.In other words, the optimum mean value of working current can beautomatically selected under the particular machining conditions withthe result that the machining efficiency becomes a maximum.

If the number of changes in pause time is larger, there is provided apause time approximating more the optimum value. Also since the electricdischarge machining operation is generally performed for a long timewith respect to one workpiece the optimum machining conditions willrelatively slowly change. Therefore, the predetermined time interval ofT as above described may be preferable long. For example, it may be inthe order of minutes.

Referring now to FIG. 3, there are illustrated various waveforms ofdischarge voltage and current developed between a workpiece and aworking electrode and resulting from variation in the machiningconditions. It is seen in FIG. 3 that each of the first three waveformsof voltage V shown on the upper portion each includes a no-load voltageportion 41 followed by an arc voltage portion 42. Also each of thewaveforms is followed by a pause time such as shown at 44 in FIG. 3.FIG. 3 also shows the remaining voltage waveforms each including onlythe arc voltage. The reference numeral 46 designates waveforms ofcurrent I corresponding to those voltage waveforms illustrated directlyabove the same. The noload voltage portion 41 is developed across theworking gap when no electric discharge occurs across the gap while thearc voltage portion 42 is developed across the gap when an electricdischarge occurs across the latter. The occurrence of the no-loadvoltage portion 41 depends upon the dimension of the working gap andwith the gap wide enough to prevent the occurrence of electricdischarge, only the no-load portions are developed across the gap. Onthe contrary, the working gap being very small in dimension causes thearc voltage portions to be more developed thereacross.

Also the no-load voltage portion may tend to disappear in accordancewith the particular maching conditions but not much dependent upon thedimension of the working gap. Eventually only the arc voltage portionsremain developed across the gap leading to the abnormally machiningstate. Under the abnormal state only a considerably increase indimension of the working gap can prevent the no-load voltage portionsfrom disappearing. On the other hand, the occurrence of those voltagewaveforms including the no-load voltage portion may be called thenormally machining state.

Therefore the presence or the absence of the no-load voltage portion candetermine the normal or abnormal state respectively. For example, if thestate in which the no-load voltage portions occurs continues for apredetermined interval of time such as T previously described inconjunction with FIG. 2, it can be determined that the normal state ispresent in the working gap. On the other hand, if the no-load voltageportions do not continuously occur for a predetermined interval of timet as shown in FIG. 3 the gap can be determined to be in its abnormalstate. In other words, a predetermined number of successive pulsevoltages including the respective no-load voltage portions representthat the gap is in its normal state while a predetermined number ofsuccessive pulse voltages including only the respective arc voltageportions represent the abnormal state of the gap.

Referring now to FIG. 4, there is illustrated a circuit diagrams of oneform of the state sensor and pulse generator 38 and 40 respectivelyshown in FIG. 1 and operative as above described. Within dotted block 50there are disposed the workpiece l4 and the working electrode 16 asshown in FIG. 1 across which is connected a potentiometer 52 having anintermediate tap thereon coupled through a Zener diode 54 to an NPN typetransistor 56 at the base electrode. The transistor 56 includes anemitter electrode connected to the workpiece l4 and a collectorelectrode connected to an emitter electrode E of a unijunctiontransistor 58. The transistor 58 includes a pair of base electrodes I3and B and a capacitor 60 is connected across the collector and emitterelectrodes of the transistor 56. The emitter electrode E of theunijunction transistor 58 is connected through a resistor 62 to apositive conductor L, leading to a DC source Ba and the first baseelectrode B thereof is also coupled to the conductor L The second baseelectrode B of the transistor 66 is connected by a resistor 64 to anegative conductor L connected to the source Ba and further coupled to abase electrode of an NPN type transistor 66. The transistor 66 includesan emitter electrode directly connected to the conductor L and acollector electrode connected to the conductor L through a relay R1. Therelay R1 includes a set of normally closed contacts Rla connected acrossthe conductors L and L through a clock pulse generator Cl, and a set ofnormally open contacts Rlb connected across the conductors L, and Lthrough a forward solenoid SWF, for a rotary switch as will be describedlater. The source Ba has connected thereacross a set of normally opencontacts Cla and a reverse solenoid SWR for the same switch seriallyconnected to the contacts Cla.

The potentiometer 52 has the intermediate tap disposed thereon in such aposition that, with the no-load current portion 50 as shown in FIG. 3developed across the machining gap between the electrode and workpiecel6 and 14 respectively, the tap provides a voltage sufficient to firethe Zener diode 54 to supply a base current to the transistor 56 therebyto render it conductive. However the occurrence of the arc voltage suchas shown at 42 in FIG. 3 across the working gap causes the tap toprovide a voltage sufficient to maintain the Zener diode 62 andtherefore the transistor 64 non-conductive. If the transistor 56 is inits nonconductive state the capacitor 60 is charged from the source Bathrough the re sistor 62. When the voltage on the capacitor 60 hasreached a predetermined fixed magnitude determined by thecharacteristics of the unijunction transistor 58, the capacitordischarges to provide an impulsive voltage or pulse across the resistor64. The impulsive voltage is applied to the transistor 66 to fire itleading to the energization of the relay R1 from the source Ba.

An interval of from the beginning of charge up to the beginning ofdischarge of the capacitor 60 determines the time constant of thecapacitor and resistor 60 and 62 respectively. It is now assumed thatthe interval just described is preselected to equal the time t as shownin FIG. 3. Under the assumed condition, if the absence of the no-loadvoltage portions 40 continues for that time t then the relay R1 isenergized to open the closed contacts Rla and close the open contactsRlb. The opening of the relay contacts Rla disconnects the clockgenerator Cl from the positive conductor L That is, the clock generatorCl is reset to its inoperative position. When the relay R1 is in itsde-energized position, the clock generator C1 is operated to generate atrain of clock pulses with a repetition period of T The generator Clprovides a contact closure type output to close its contacts C 1a topermit the solenoid SWR to be energized. It is noted that what is timedout by block 110 shown is FIG. 2 is equal to that pulse repetitionperiod of T The solenoids SWF, and SWR are operatively associated with arotary switch SW1 including and a plurality of stationary contactsselectively engaged by a movable arm and connected to a plurality ofcapacitors 68a, b, c, n different in capacitance from one another forthe purpose as will be apparent hereinafter. Preferably the capacitorsprogressively increase in capacitance from the capacitor 68a toward thecapacitor 68n. The forward solenoid SWF responds to each pulse appliedthereto to step the switch SW1 in the forward direction or in thecounterclockwise direction as viewed in FIG. 4 whereby the movable armis connected to that capacitor having a higher capacitance. On the otherhand, the reverse solenoid SWR, responds to each pulse applied theretoto step the switch SW1 in the reverse direction or in the clockwisedirection. This causes the movable switch arm to engage that capacitorless in capacitance.

FIG. 4 also shows an astable multivibrator of conventional constructiongenerally designated by the reference numeral 70. The multivibrator 70includes a pair of NPN type transistors 72a and b having emitterelectrodes connected together to the negative conductor L and collectorelectrodes coupled together to the positive conductor L The collectorelectrode of the transistor 72a is connected through a capacitor 74 tothe base electrode of the transistor 72b and also through the conductorL through a resistor 76. The base electrode of the transistor 72a isconnected to the conductor L,

through a resistor and also directly connected to the movable arm of theswitch SW1. The collector electrode of. the

transistor 72b is connected to all the capacitors 68a, b, n and providesthe output of the pulse generator 40 connected to the amplifier 42.

The astable multivibrator 70 produces a train of pulses having a pulsewidth determined by the time constant of the capacitor 74 and resistor76 and a pause time determined by the time constant of the resistor 78and that capacitor 68 connected thereto through the movable arm of theswitch SW1.

While the multivibrator 70 is shown in FIG. 4 as including the singlecapacitor 74 it is to be understood that the capacitor 74 may be repacedby a plurality of parallel capacitors adapted to be selectivelyconnected between the collector and base electrodes respectively of thetransistors 72a and b. If desired, several tens of such capacitors maybe employed. It is also to be understood that the switch SW1 can steponly along a predetermined number of the stationary contacts in eitherof the forward and reverse direction and is prevented from rotatingbeyond the end contacts in response to a further pulse or pulses appliedto either of its solenoids SWF and SWR,.

The arrangement as illustrated in FIG. 4 is operated as follows: If themachining state becomes abnormal then relay R1 is energized to close thecontacts Rlb as above described. The closure of the contacts Rlb causesthe forward solenoid SWF to be energized to step forwardly the switchSW1 by one position whereupon the movable arm thereof disengages fromthat capacitor 68 previously engaged thereby and engages the adjacentcapacitor higher in capacitance. This cause the pause time to increaseby a predetermined incremental amount. The relay R1 is deenergized uponcompleting discharge of the capacitor 60. The opening of the contactsRla ensures that the clock generator C1 is in its inoperative Rposition. If the abnormal state under which the no-load voltage portions40 are not developed across the working gap 18 continues within theabovementioned time interval of t, after the completion of discharge ofthe capacitor 60, the relay R1 is again energized to further increasethe pause time by a predetermined incremental amount in the same manneras above described. In this way the pause time increases by thepredetermined incremental amounts until the abnormal state disappears.

Once the normal-state has been restored the relay R1 is deenergized toclose its contacts Rla. Then if the normal state continues for theabovementioned time interval of T the clock generator Cl provides anoutput or a clock pulse to close its contacts Cla to permit theenergization of the reverse solenoid SWR Therefore, the switch SW1 stepsreversely by one position to engage the movable arm thereof with thatcapacitor 68less in capacitance resulting a decrease in pause time bya'predetermined amount. A further continuation of the normal state forthe interval of T after the pause time has increased causes similarlythe switch SW1 to step reversely by one position whereby the pause timefurther decreases by a predetermined amount.

In this way, the pause time is adjusted by the predetermined amounts sothat the mean working current is controlled to its optimum value inaccordance with the particular machining state.

It is understood that the predetermined amounts by which the pause timeis adjusted are not required to be equal to one another.

In the event that the abnormal state does not disappear only through anincrease in pause time by the single incremental amount, the arrangementof FIG. 4 may be operated to increase the pause time by two or moreincremental amounts even though the single incremental amount of thepause time would have been actually optimum upon reaching the normalstate. This results in time being wasted until the optimum value isrestored.

FIG. 5 illustrates a modification of the invention efi'ective fordecreasing the waste of time just described. The arrangement illustratedis operative to provide a maximum possible pause time once in the eventof varying the pause time by a single, relatively large amount inresponse to the occurrence of the abnormal state and then to'decreasethe pause time stepwise at predetermined time intervals to a pause timelonger by a single predetermined amount than that pause time with whichthe abnormal state occurred. In addition to the state-sensing circuit 50and the pulse generator 40 shown in FIG. 4, the arrangement comprises, arelay R2 connected through normally open contacts Rlc of the relay R1and normally closed contacts R4a of a relay R4 across the positive andnegative conductor L and L with holding, normally open contacts R2bthereof parallel to the contacts Rlc. A pair of relays R3 and R4 areconnected in parallel circuit relationship and between the conductors Land I. through serially connected rotary switches SW2 and SW3. Theswitches SW2 and SW3 each include a plurality of stationary contacts 8,,S S S, selectively engaged by its own movable arm with each of thecontacts of one of the switches connected to the corresponding contactof the other switch. The switch SW2 is arranged to interlock with theabovementioned rotary switch SW1.

In addition to the abovementioned clock generator C1 connected acrossthe conductor L, and L through the normally closed contacts Rla, a pairof clock pulse generators C2 and C3 are connected across'the conductorsL and L through normally open contacts R2c of the relay R2 and seriallyconnected, normally closed contacts R2a and R3a of the relays R2 and R3respectively. The clock generators C1, C2 and C3 generate trains ofpulses having repetition periods of T,, T and T respectively.

In order to step the switch SW2 in either of the forward and reversedirections, forward and reverse solenoids SWF and SWR are connectedacross the conductors I. and L through normally open contacts C2a andC3a of the clock generators C2 and C3 respectively while solenoids SWFand SWR are similarly connected through normally open contacts Rld andClb of the relay R1 and generator C1 to step the switch SW3 in either ofthe forward and reverse directions. It is again assumed'that the forwarddirection is directed from the contact S toward the contact 5,,corresponding to a maximum pause time. Those solenoids are identical inoperation to the solenoids SW1- and SWR as shown in FIG. 4.

The operation of the arrangement as above described will now bedescribed with reference to FIG. 6 wherein there are various waveformsdeveloped in the arrangement or the operative position of the componentsR1, R2, R3, C1, C2, C3, SW2 and SW3. In FIG. 6, those components areshown as being at predetermined high levels when energized and at zerolevel when de-energized.

Now assuming that the working gap is in its normal state, the switchesSW2 and SW3 are at the same contact position S Then if the gap isbrought into its abnormal state at a time point of t (see FIG. 6) therelay R1 is energized as above described in conjunction with FIG. 3.Thereby the contacts R11: are closed to permit the relay R2 to beenergized. The energization of the relay R2 causes the clock generatorC2 to be started through the closure of the contacts R2c. Then thecapacitor 60 (see FIG. v4) has been discharged whereupon the relay R1 isreset to its inoperative position. It is noted that FIG. 6 illustratesthe operation of the arrangement as shown in FIG. 5 on the assumptionthat the abnormal state has been removed after that resetting of therelay R1. When energized. the clock generator C2 is operated tointermittently close the contacts C2a with the period of T (see (f) inFIG. 6) to cause the forward solenoid SWF to be intermittently energizedwith the result that the interlocking switches SW1 and SW2 step to thecontact S to provide a maximum pause time. That is, a time inverval oft(see FIG. 6) has elapsed from the time point t, at which the abnormalstate occurred until the contact 8,, is reached (see (h) in FIG. 6).

On the other hand, the energized relay RI closes its contacts Rld tocause the energization of the forward solenoid SWF of the switch SW3whereby the switch SW3 steps by one position reaching the contact 8,,(see (i) in FIG. 6). When the switch SW2 has reached the contact 8,, ata time point of t or the end of the time interval of z, the relay R4 isenergized to open its contacts R4a leading to a de-energization of therelay R2. As a result, the clock generator C2 ceases to be operated andinstead the clock generator C3 begins to be operated to intermittentlyclose its contacts C3a with the period of T (see (g) in FIG. 6). Thiscauses the reverse solenoid SWR to be energized at time intervals of Tto step the rotary switches SW1 and SW2 in the reverse direction. Then atime interval of (see FIG. 6) has elapsed from the time point of t to atime point of I at which the switches SW2 and SW3 are assumed to reachthe contact S together. Therefore the relay R3 is energized to open itscontacts R3a thereby to stop the operation of the clock generator C3.The present contact S,, leads the previous contact S, by one position.Thus the pause time has increased by a single predetermined incrementalamount.

Thus it will be appreciated that the abnormal state is effectivelyeliminated during the abovementioned increase in pause time to itsmaximum value corresponding to the contact S,,, and during the reversionof the pause time to the contact S,,. as above described. Thiseliminates the necessity of consuming a long time for setting thesucceeding pause time and also causes a decrease in waste of control.

It is now assumed that the working gap is held in its normal state forthe time interval of T after the relay R1 has been reset to itsinoperative position. Then the clock generator Cl provides an output(see (c) in FIG. 6) to close its contacts Clb to step the switch SW3 inthe reverse direction by one position while at the same time the relayR3 is de-energized. The deenergization of the relay R3 causes itscontacts C3a to be closed to operate the clock generator C3. Thereforethe generator C3 provides its output to close its contacts C3a to causethe switches SW1 and SW2 to reversely step by one position with theresult that the pause time decreases by a single predetermined amount.

FIG. 7, wherein the axis of ordinates represents a current and the axisof abscissas represents time, illustrates the mean machining currentvaried during the control of the pause time as above described inconjunction with FIGS. and 6.

It has been found that the pulse repetition periods of T and T arepreferably above 1 minute and of from 1 to 10 seconds respectively whileT is selected to be under I second so as to provide to the switches astepping speed as high as possible because they are required to berapidly operated. However, it is to be understood that the fingers justspecified of the repetition periods are not necessarily definitive. Thisis because it may be better to vary these figures in accordance with thematerials of the workpiece and the working electrode, the type of boreto be machined such as through and bottomed bores, etc.

FIG. 3 shows the results of experiments conducted with a practicalapparatus according to the invention and also a working electrode 80used in the experiments. As shown, the electrode 80 was formed ofgraphite into a square cross-section having one side mm long. Curves 82and 84 plot the mean working current i in amperes (in ordinates) againsta machining time in minutes (in abscissas) and curves 86 and 88 plot adepth of bore d in millimeters as a function of the machining time.Curves 82 and 86 indicate the invention while curves 84 and 88 indicatethe prior-art practice.

From FIG. 8 it is seen that the invention finely selected the meanworking current with an increase in depth of bore as compared with theprior art practice. As a result, the machining speed is high andtherefore the machining time decreases.

Further it is to be noted that the fine selection of the mean workingcurrent is automatically accomplished. Once the fundamental machiningconditions such as the peak value and pulse width of a discharge currentinvolved provided by desired roughness of a surface to be finished, aratio of consumption of an electrode used to an amount of workpiecesmaterial removed determined for the particular purpose have been presetthe machining is continuously efiected with a high degree of machiningefficiency without the necessity of effecting any manual adjustmentafter the machining conditions have been set and also without the skillfulness of the operator.

What we claim is:

1. An electric discharge machining apparatus comprising, means forholding a workpiece and a working electrode disposed in opposedrelationship to said workpiece to form a working gap therebetween, meansincluding said electrode for applying a voltage across said gapcomprising a switching element for intermittently interrupting the powersupply to said working gap to apply across said working gap pulsevoltages each followed by a pause time which is variable, means forvariably adjusting individual pause times in dependence upon workingvoltages across said working gap, a second switching element responsiveto no occurrence of electric discharge across said working gap due tothe application of said pulse voltages thereacross in one operativeposition and responsive to both the occurrence of electric dischargeacross said working gap due to the application of said pulse voltagesthereacross and the presence of said pause time in another operativeposition, and said adjusting means comprising means responding to saidanother operative position of said second switching element to variablyadjust said pause time.

2. An electric discharge machining apparatus comprising, means forholding a workpiece and a working electrode disposed in opposedrelationship to said workpiece to form a working gap therebetween, meansincluding said electrode for applying a voltage across said gapcomprising, switching element for intermittently interrupting the powersupply to said working gap to apply across said working gap pulsevoltages each followed by a pause time, means for adjusting said pausetime in accordance with voltages across said working gap, a secondswitching element responsive to no occurrence of electric dischargeacross said working gap due to the application of said pulse voltagesthereacross in one operative position and responsive to both theoccurrence of electric discharge across said working gap due to theapplication of said pulse voltages thereacross and the presence of saidpause time in another operative position, and actuating means responsiveto the consecution of a predetermined number of the pulse voltagesputting said second switching element in the one operative position tocontrol said adjusting means to decrease said pause time.

3. An electric discharge machining apparatus comprising means forholding a workpiece and a working electrode disposed in opposedrelationship to said workpiece to form a working gap therebetween, meansincluding said electrode for applying a voltage across said gapcomprising a switching element for intermittently interrupting the powersupply to said working gap to apply across said working gap pulsevoltages each followed by a pause time, means for adjusting said pausetime in accordance with voltages across said working gap, a secondswitching element responsive to no occurrence of electric dischargeacross said working gap due to the application of said pulse voltagesthereacross in one operative position and responsive to both theoccurrence of electric discharge across said working gap due to theapplication of said pulse voltages thereacross and the presence of saidpause time and operable to another operative position, and actuatingmeans responsive to the sequence of a predetermined number of the pulsevoltages operating said second switching element to said anotheroperative position to control said adjusting means to increase saidpause time.

4. An electric discharge machining apparatus comprising, means forholding a workpiece and a working electrode disposed in opposedrelationship to said workpiece to form a working gap therebetween, meansincluding said electrode for applying a voltage across said gapcomprising a switching element for intermittently interrupting the powersupply to said working gapto apply across said working gap pulsevoltages each followed by a pause'time, means for adjusting said pausetime in accordance with voltages across said working gap, a secondswitching element responsive to no occurrence of electric dischargeacross said working gap due to the application of said pulse voltagesthereacross in one operative position and responsive to both theoccurrence of electric discharge across said working gap due to theapplication of said pulse voltages thereacross and the presence of saidpause time to be put in another operative position, first actuatingmeans responsive to a sequence of a predetermined number of the pulsevoltages operating said second switching element to the one operativeposition to control said adjusting means to decrease said pause time andsecond actuating means responsive to a sequence of a predeterminednumber of the pulse voltages operating said second switching element tosaid another operative position to control said adjusting means toincrease said pause time.

5. An electric discharge machining apparatus comprising, means forholding a workpiece and a working electrode disposed in opposedrelationship to said workpiece to form a working gap therebetween, meansincluding said electrode for applying a voltage across said gapcomprising a switching element for intermittently interrupting the powersupply to said working gap to apply across said working gap pulsevoltages each followed by a pause time, means for adjusting said pausetime in accordance with voltages across said working gap, a secondswitching element responsive to no occurrence of electric dischargeacross said working gap due to the application of said pulse voltagesthereacross in one operative position and responsive to both theoccurrence of electric discharge across said working gap due to theapplication of said pulse voltages thereacross and the presence of saidpause time in another operative position, first actuating meansresponsive to a sequence of a predetermined number of the pulse voltagesputting said second switching element in said one operative position tocontrol said adjusting means to decrease said pause time, and secondactuating means responsive to a sequence of a predetermined number ofthe pulse voltages putting said second switching element in said anotheroperative position to control said adjusting means to increase saidpause time, and said adjusting means having means responding to each ofthe operations of either of said first and second actuating means to becontrolled by a predetermined amount.

6. An electric discharge machining apparatus comprising, means forholding a workpiece and a working electrode disposed inopposedrelationship to said workpiece to form a working gaptherebetween, means including said electrode for applying a voltageacross said gap comprising a switching element for intermittentlyinterrupting the power supply to said working gap to apply across saidworking gap pulse voltages each followed by a pause time, means foradjusting said pause time in accordance with voltages across saidworking gap, a second switching element responsive to no occurrence ofelectric discharge across said working gap due to the application ofsaid pulse voltages thereacross in one operative position and responsiveto both the occurrence of electric discharge across said working gap dueto the application of said pulse voltages thereacross and the presenceof said pause time in another operative position, first actuating meansresponsive to holding of said second switching element in said anotheroperative position for a predetermined time interval to control saidadjusting means to increase said pause time, and second actuating meansresponsive to an increase in pause time to a predetermined value throughthe operation of said first actuating means to control said adjustingmeans to decrease said pause time.

7. An electric discharge machining apparatus comprising,

means for holding a workpiece and a working electrode disposed inopposed relationship to said workpiece to form a working gaptherebetween, means including said electrode for applying a voltageacross said gap comprising a switching element for intermittentlyinterrupting the power supply to said working gap to apply across saidworking gap pulse voltages each followed by a pause time, means foradjusting said pause time in accordance with voltages across saidworking gap, a second switching element responsive to no occurrence ofelectric discharge across said working gap due to the application ofsaid pulse voltages thereacross in one operative position and responsiveto both the occurrence of electric discharge across said working gap dueto the application of said pulse voltages thereacross and the presenceof said pause time in another operative position, first actuating meansresponsive to holding of said second switching element in said anotheroperative position for a predetermined time interval to control saidadjusting means to increase said pause time, and second actuating meansresponsive to an increase in pause time to a predetermined value throughthe operation of said first actuating means to control said adjustingmeans to decrease said pause time, an amount by which the pause timeincreased by said first actuating means is larger than an amount bywhich the pause time is decreased by said second actuating means.

1. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time which is variable, means for variably adjusting individual pause times in dependence upon working voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time in another operative position, and said adjusting means comprising means responding to said another operative position of said second switching element to variably adjust said pause time.
 2. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising, switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time in another operative position, and actuating means responsive to the consecution of a predetermined number of the pulse voltages putting said second switching element in the one operative position to control said adjusting means to decrease said pause time.
 3. An electric discharge machining apparatus comprising means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross In one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time and operable to another operative position, and actuating means responsive to the sequence of a predetermined number of the pulse voltages operating said second switching element to said another operative position to control said adjusting means to increase said pause time.
 4. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time to be put in another operative position, first actuating means responsive to a sequence of a predetermined number of the pulse voltages operating said second switching element to the one operative position to control said adjusting means to decrease said pause time and second actuating means responsive to a sequence of a predetermined number of the pulse voltages operating said second switching element to said another operative position to control said adjusting means to increase said pause time.
 5. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time in another operative position, first actuating means responsive to a sequence of a predetermined number of the pulse voltages putting said second switching element in said one operative position to control said adjusting means to decrease said pause time, and second actuating means responsive to a sequence of a predetermined number of the pulse voltages putting said second switching element in said another operative position to control said adjusting means to increase said pause time, and said adjusting means having means responding to each of the operations of either of said first and second actuating means to be controlled by a predetermined amount.
 6. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric disCharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time in another operative position, first actuating means responsive to holding of said second switching element in said another operative position for a predetermined time interval to control said adjusting means to increase said pause time, and second actuating means responsive to an increase in pause time to a predetermined value through the operation of said first actuating means to control said adjusting means to decrease said pause time.
 7. An electric discharge machining apparatus comprising, means for holding a workpiece and a working electrode disposed in opposed relationship to said workpiece to form a working gap therebetween, means including said electrode for applying a voltage across said gap comprising a switching element for intermittently interrupting the power supply to said working gap to apply across said working gap pulse voltages each followed by a pause time, means for adjusting said pause time in accordance with voltages across said working gap, a second switching element responsive to no occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross in one operative position and responsive to both the occurrence of electric discharge across said working gap due to the application of said pulse voltages thereacross and the presence of said pause time in another operative position, first actuating means responsive to holding of said second switching element in said another operative position for a predetermined time interval to control said adjusting means to increase said pause time, and second actuating means responsive to an increase in pause time to a predetermined value through the operation of said first actuating means to control said adjusting means to decrease said pause time, an amount by which the pause time increased by said first actuating means is larger than an amount by which the pause time is decreased by said second actuating means. 